The present invention relates to a process for the preparation of hydrated or non-hydrated, metal fluorides more particularly usable for the production of fluoride glasses.
More specifically, it relates to a process for the continuous preparation of metal fluorides which can easily be separated from the reaction medium and it more particularly applies to the preparation of fluorides usable for the production of fluoride glasses, such as fluorides of zirconium, aluminium, rare earths, such as lanthanum, and alkaline earth metals, such as barium.
Various processes are known for the preparation of metal fluorides, but they are not generally suitable for the continuous production of fluorides, because the fluoride obtained is difficult to separate from the reaction medium.
Thus, U.s. Pat. No. 2 639 218 describes a process for the productin of anhydrous zirconium fluoride consisting of reacting the zirconium oxide or zirconyl chloride with an aqueous hydrofluoric acid solution, then heating the reaction medium to dry it and calcining the dry product obtained. Thus, in this process it is necessary to completely dry the reaction medium to recover the product and this does not permit the continuous production of zirconium tetrafluoride.
French Pat. No. 2 059 672 illustrates a process for the preparation of zirconium tetrafluoride consisting of reacting a zirconyl nitrate solution in nitric acid with hydrofluoric acid operating at a temperature of at least 30.degree. C., then separating the monohydrated zirconium tetrafluoride and drying the same.
This preparation mode can be adapted to the continuous production of zirconium tetrafluoride, but it requires the use of a relatively complex assembly for the preparation of the zirconyl nitrate solution in nitric acid and for the separation of the products.
U.S. Pat. No. 2 789 882 illustrates the preparation of zirconium tetrafluoride by the reaction of zirconium tetrachloride in suspension in an organic liquid with gaseous hydrofluoric acid. This gives a zirconium tetrafluoride precipitate, but the latter can be polluted by the solid reagent (ZrCl.sub.4) in suspension. Moreover, it is not possible to obtain a fine precipitate of ZrF.sub.4.